California Study Finds No Link Between Vaccines, Autism

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California Study Finds No Link Between Vaccines, Autism

The mercury-containing vaccine additive thimerosal is not a primary cause of autism, says a study published yesterday in the Archives of General Psychiatry.

High doses of thimerosal were used throughout the 1990's in infant vaccines before being largely removed from U.S. supplies in 1999. Mercury is a potent neurotoxin, and some people have blamed it for the dramatic, tragic rise of autism in the United States.

Yesterday's study, authored by California Department of Public Health researchers Robert Schechter and Judith Grether, used California Department of Developmental Services data to track rates of autism diagnoses since thimerosal's removal. If thimerosal was responsible for the autism epidemic, there would ostensibly have been a drop in diagnoses in children born after the 1999 removal – but that's not what they saw. The numbers continued to rise.

The CDDS data is considered a gold standard for autism epidemiology, and the study is more convincing than a CDC study published in September. It may be the most conclusive argument yet that thimerosal and autism are not strongly, or even moderately, connected.

However, that isn't reason to consider the investigation a waste of time prompted by the "mercury militia," as thimerosal-autism link advocates are deregatorily dismissed. The anonymous RespectfulInsolence blogger says the connection "was never particularly scientifically plausible in the first place; it was just barely plausible enough that activists with an agenda could make scientists throw up their hands and say, 'All right, let's take a look.'" The criticism echoes that of UCLApharmacologist Paul Krugstad, who after the CDC study said, "The removal of thimerosal created the impression of risk, where none existed."

But the thimerosal-autism link was plausible, at least in the lab:studies found that the preservative damaged cell cultures and mice. I have vivid memories of being shown a video by Columbia University epidemiologist Mady Hornig, who gave mice thimerosal doses proportionate to those received by human babies, then watched as the mice exhibited autism-like symptoms, particularly repetitive behavior. In some instances the mice repeated their customary grooming habits until they'd chewed holes in each other.

The doses received by children were several times the EPA's recommended maximums; those maximums were rules of thumb rather than precise scientific markers, but they were guidelines nonetheless. Allowing thimerosal doses to increase through the 1990s as the routine infant immunization schedule expanded was extraordinarily careless. Vaccine manufacturers and public health officials dodged a bullet – and it's still not clear, and might not ever be clear, whether thimerosal might have caused autism in a statistically insignificant number of children unable to metabolize mercury.

The autism-thimerosal link appears, for public health purposes, to be dead. But rather than mocking the understandable anger of confused parents trying to grapple with early and imperfect science, critics ought to be grateful that vaccine carelessness didn't wreak havoc on the mental development of a generation.Continuing Increases in Autism Reported to California's Developmental Services System [Archives of General Psychiatry]